This invention relates generally to separation of very large particles from a fluid borne stream of relatively fine particles by screening, and more particularly to coarse screening devices for washing and draining fine fiber/liquid suspension away from coarse nodules and/or other large particles.
For example in the digestion of wood for pulpmaking, a small fraction of chips become masked by other chips or are sufficiently digestion resistant to survive the digestion process and are commonly called knots. These and other undigested particles must be removed from the fluid borne pulp stream to prevent clogging of processing equipment and, ultimately, degradation of paper quality.
Removal of knots is normally accomplished in a knotter which screens the process slurry to remove them. A significant quantity of acceptable pulp is discharged along with the knots being rejected. This pulp must be separated from the knots before the knots are reprocessed or otherwise disposed of. In most cases, separation is accomplished in a knot drainer, which is a coarse screen which separates knots from pulp fibers and discharges the knots in a relatively dry and fiber free condition.
"Secondary" knot drainers, commonly consist of either high speed vibratory screens or generally vertical screw drainers. These may permit air entrainment with consequent foam generation which can adversely affect the process and require excessive defoamer consumption. In the screw type knot drainers, relative motion by the conveying screw and the screen plate can cause size reduction of the suspended particles. This "comminution" of knots can result in fibrous and resinous debris which is difficult to remove in downstream processing and which can degrade paper quality. Another consequence of using either type of secondary knot drainer may be discharge of an excessive amount of fiber with the knots. This fiber must either be recovered in further processing or be lost to production. Because of vibration and wear, maintenance costs for repair and replacement of screens and other components as well as lost production due to downtime for repairs can be unacceptably high. These and other disadvantages can reduce the efficiency of the knot removal and knot draining operation and hence increase the cost of producing clean pulp.
The foregoing illustrates limitations known to exist in present screening devices for removing coarse particles from a liquid borne fine particle slurry such as the various pulp types used in papermaking. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.